Silica alkoxides

The hydrolysis of silica alkoxides is a very slow reaction, a sharp acceleration is observed when the pH is shifted to acidic or alkaline side. This is caused by catalysis by protons or hydroxy groups acting as strong electrophilic or nucleophilic agents, respectively [8,18,58],... 

The use of silica particles in bioapplications began with the publication by Stober et al. in 1968 on the preparation of monodisperse nanoparticles and microparticles from a silica alkoxide monomer (e.g., tetraethyl orthosilicate or TEOS). Subsequently, in the 1970s, silane modification techniques provided silica surface treatments that eliminated the nonspecific binding potential of raw silica for biomolecules (Regnier and Noel, 1976). Derivatization of silica with hydrophilic, hydroxylic silane compounds thoroughly passivated the surface and made possible the use of both porous and nonporous silica particles in all areas of bioapplications (Schiel et al., 2006). 

Prehydrolysis method. Synthesis using Si02 instead of silica alkoxides. Gel composition SiO2 vTiO2 0.4TPAOH 35H2O 0 < x < 0.03... 

These condensation reactions work not only for silica alkoxides, but also for most of the metal alkoxide precursors. The photolysis of alcohols leads to bond... 

Fig. 6. Dependence of stmctural density of alkoxide-derived porous silica gels as a function of sintering temperature held for 12 h for three different...

The synthesis of Ti-Si-TUD-1 is analogous to the silica version a portion of the reactant is a titanium alkoxide, such as titanium (IV) n-butoxide. One of the early comparative catalytic tests of TUD-1 versus MCM-41 was for cyclohexene epoxidation. 

A combination of DAT and a metal alkoxide other than titanium alkoxide serves as a poor catalyst for the epoxidation of allylic alcohols. However, the combination of DAT and silica-supported tantalum alkoxides (2a) and (2b) prepared from Ta(=CHCMe3)(CH2Cme3)3 and silica(5oo) shows high enantioselectivity in the epoxidation of E-allylic alcohols, though chemical yields are not very great (Scheme 4).3... 

The synthesis of group 4 alkoxide complexes grafted on the surface of silica illustrates this approach.44-47 Two routes were considered, which are schematically represented in Equations(2) and (3) they differ in the nature of the precursor complex, which is either the tetra-alkoxide M(OR)4 (Equation(2)), or the tetra-alkyl complex MR4, M = Ti, Zr, Hf (Equation(3)) ... 

The second approach (Equation(3)) has a number of advantages over the first one (Equation(2)). The alkyl complexes are more reactive than the related alkoxides, the latter being for group 4 elements generally associated into dimers or trimers 48 also, reaction (2) liberates an alcohol which may further react with the surface of silica, whereas the alkane ( Equation(3)) is inert. It was demonstrated by various spectroscopic techniques and elemental analysis that with a silica dehydroxylated at 500 °C under vacuum, the stoichiometry of reaction (3) corresponds to n = 1.45,46 Moreover, a better control of the surface reaction was achieved with the procedure represented in Equation(3). 

The more hindered alkoxide Ti(OiPr)4 was used as the precursor complex with surface silanols of an amorphous silica support this reaction is reported to lead to the same environment of Ti as in TS-1, but only when the reaction is carried in cyclohexanol as the solvent. Epoxidation of octene, cyclohexenol, and norbornene with FI202 in phenylethanol leads to 95-98% epoxide selectivity.147... 

The zirconium alkoxides Zr(OR)4 are reported as inactive for the epoxidation of olefins under the conditions recommended with the titanium analogs. When synthesized by reaction of Zr(CH2CMe3)4 with silica, followed by hydrolysis or calcination, a solid as active as the related Ti-based catalyst is obtained. The low selectivity for the formation of the epoxide is related to the fact that the same Zr centers catalyze both the formation and the decomposition of the epoxide.46... 

The use of alkoxides for fabrication of silica materials has some advantages that will be discussed later. They serve as a precursor a silicic acid is generated in the course of their hydrolysis that thereafter enters into condensation reactions (1). The main processes may be presented by Scheme 3.1. 

Fig. 3.3 The common two-stage sol-gel process used to entrap biopolymers in a silica matrix (see Scheme 3.1). The first stage serves to hydrolyze alkoxide Equation (2) in the acidic or alkaline media. This is also attended with condensation reactions Equations (3) and (4) resulting in the formation of oligomeric silica that self-organizes in the form of sol nanoparticles. Biopolymers are entrapped in the...

In a procedure developed by Carturan et al. [79,93] the alcohol is removed in a gas flow. This is also used to deliver the silica precursor. An alkoxide is initially evaporated to saturate a heated inert gas which serves as a carrier. When the gas... 

The realization of the reasons for poor biocompatibility of general alkoxides with biopolymers led to the development of approaches to minimize or eliminate the problem of the detrimental effect of alcohols. This can be done in two ways modification of the sol-gel processing or the silica precursor. This is considered in some detail below. 

An example of the appropriate application of organically-modified silica precursors is alkoxides with an alkyl group. When methyltrimethoxy- or methyl-triethoxysilane (Figure 3.2) was added in formulations to increase the hydro-phobicity of ORMOSILs, it resulted in a better enzymatic activity of lipases immobilized in the alkyl-modified silica than in a hydrophilic matrix fabricated by means ofTEOS alone [51,80,129-133]. Similarly, an increased stability of lipase from Candida antarctica B was observed after its immobilization in a silica matrix... 

Audebert, P. and Sanches, C. (1994) Modified electrodes from hydrophobic alkoxide silica gels — insertion of electroactive compounds as glucose oxidase. Journal of Sol-Gel Science and Technology, 2, 809-812. 

The sol-gel technology, based on various alkoxides, allows production of classical silica glasses, as well as multicomponent materials, merging silicates with titanates, borates and a variety of other oxides (Zn, La, Al, Li, B, K, etc.). The alkoxide gel method can be also used for production of certain non-silicate oxide glass-like materials (e.g., ZrCV, etc.)25. 

In recent years, silica sol-gel-based inorganic-organic hybrid materials have also been reported. The introduction of various functional groups into organic alkoxide has led to organically modified sol-gel glasses (ormosils). Some of the ormosil monomers and ormosil formations can be found in Fig. 16.3. Redox molecules can be coupled with the ormosil monomer functional group. The immobilized redox molecules can... 

These reactions have been studied in detail for materials such as silica, and understanding of reaction mechanisms, as well as of the role of the precursor and catalyst (acid or base), has been well documented.63,64 Similar studies have been carried out in other material systems, most notably, lead zirconate titanate [Pb(Zr,Ti)03 PZT].52,65-68 For multicomponent (mixed-metal) systems such as those noted, prehydrolysis of less reactive alkoxides is sometimes employed to improve solution compositional uniformity. Other synthetic strategies to achieve molecular level mixing of reagents have also been employed. Here, synthesis of mixed-metal alkoxides has been a focus of investigators.40-42 A key point is to restrict the amount of water and to control how it is added to form solubalizable precursor species, rather than to induce precipitation.1,52,69,70... 

Manipulation of the factors noted above may be used to control the characteristics of the hydrolysis and condensation reactions, which will dictate precursor nature. These reactions have been studied extensively for silica,1,63 and some similarity in chemical behavior to electronic material systems (PZT, etc.) has been noted because of the alkoxide reagents used in the synthesis of these material systems.52,70,71... 

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